PURPOSE: To determine whether retinal pigment epithelial (RPE) cells can inhibit mature dendritic cells (mDCs).METHODS: Cultured RPE cells were established from C57BL/6 mice. DCs were established from bone marrow cells of normal mice, and mDCc were induced by culture in medium containing granulocyte macrophage-colony-stimulating factor (GM-CSF) and IL-4 in the presence of lipopolysaccharide and TNF-α. Activation of mDCs was assessed by a proliferation assay and ELISA to measure the production of pro-inflammatory cytokines (TNF-α, IL-1β, and IL-12p40). Expression of major histocompatibility complex (MHC) class II, CD11c, and costimulatory molecules such as CD80, CD86, programmed cell death 1 ligand 1 (PD-L1), and PD-L2 on mDCs or RPE-exposed mDCs was evaluated by immune staining and flow cytometry. Production of IL-1 receptor antagonist (IL-1Ra) by RPE cells was evaluated by oligonucleotide microarray or ELISA. Anti-IL-1Ra neutralizing antibodies or RPE cells from IL-1Ra knockout donors were used for the assay.RESULTS: Cultured RPE cells greatly suppressed the activation of mDCs, especially the production of pro-inflammatory cytokines, and the expression of cell-surface molecules. Moreover, RPE cells significantly suppressed mixed lymphocyte reactions by mDCs. In an examination of immunoregulatory candidate molecules, RPE cells expressed much higher levels of IL-1Ra as compared with control cells, and RPE cells pretreated with recombinant TNF-α and/or IL-1β produced high levels of IL-1Ra. RPE cells in the presence of anti-IL-1Ra antibodies, but not other candidate factors, failed to suppress activation by mDCs. In addition, RPE cells from IL-1Ra null donors failed to suppress mDC activation.CONCLUSIONS: Our results suggest that ocular resident cells can produce pro-inflammatory cytokine antagonist that suppresses antigen-presenting cell activation.